Hey Dan,
GSU is a great tool to use. Easy to learn and free to use without using the Pro version.
One disadvantage is that models can become unoptimized quickly though.
A guy here explains a detailed tutorial on do's and don't regarding geometry:
http://www.crydev.net/viewtopic.php?f=315&t=71307&hilit=playup
Whatever you create with it depends on your effort and experience.
This, for example, is made in GSU:
As well as this:
Not saying one is bad. It's just to illustrate you can go anywhere with it.
Another great free tool is called Blender. This one works quite differently opposed to GSU but is more advanced and eventually will make certain things easier to create.
You might want to try out both.